Aircraft generally include an airframe, which may be regarded as an underlying skeleton, to which skin panels are attached to form a smooth aerodynamic outer surface. The wings also include underlying structure covered with skin panels. Typically, skin panels are light and thin to minimize the weight of the aircraft and increase its payload and range. Since skin panels are thin, they are generally flexible and require stiffening to prevent undesired movement, flexing and vibration during flight.
Hat stringers have been used for decades in the aerospace industry for stiffening metal fuselage sections and metal wing skins on both commercial and military aircraft. These stringers are formed of thin metal panel with acute angles that result in a trapezoidal shape. Relatively simple metal-forming techniques are used to bend the metal into the acute angles required for this shape. These metal forming techniques include brake forming or rolling the metal into the hat stringer shape. These techniques allow the production of hat stringers with tight, constant angular bends and straight or flat legs. In FIG. 1, for example, a hat stringer 10 is attached to a fuselage skin panel 20 by a series of fasteners 25 (two shown) that are attached at intervals along the leg 16 of the hat stringer 10. The hat stringer 10 is trapezoidal having a flat upper surface 12 and sloping sides 14 that are at an angle to the legs 16, which are substantially aligned with the flat upper surface 12. The intersection between sides 14 and legs 16 may be characterized as sharp, tight or having a small radius of curvature.
Composite materials that include a resin matrix and filler have found increasing application in the aerospace industry because of their relatively light weight and favorable physical properties. In general, the fillers may be reinforcing or non-reinforcing in nature and may be in a variety of shapes, for example, powders, particulates, fibers tapes of unidirectional fibers, woven fabrics, and the like. The resins are organic polymeric materials that may include for example the commonly used epoxy resins.
As composite materials (e.g., carbon fiber and epoxy matrix resin) became more prevalent in the industry for fuselage skin and wing skin panels, hat stringers remained popular for a variety of reasons. For example, while hat stringers can be made from a single stack of material, other less common shapes such as I-shaped, J-shaped, or T-shaped stiffeners require the combination of multiple stacks and radius fillers.
As stringers transitioned from metallic materials to composites, modifying the stringer shape from the straight legs and tight bends of hat stringers was not a high priority. This is largely because the structural performance of traditional hat shapes is well understood and because when creating tools in which composite hats can be cured, straight or flat shapes with relatively small radii details are simpler to manufacture.
Composite hat stringers are now used extensively in certain commercial transport aircraft. An example of a process for attaching composite stringers to a fuselage section is illustrated in FIG. 2. A hat stringer 10 rests on fuselage skin which is backed by a semi-rigid caul sheet 35. The interior of the hollow hat stringer 10 has an inflatable rubber bladder 36, and the exterior of the hat stringer 10 is covered by an inside mold line (IML) tool. The bladder 36 is inflated while the IML tool provides outside pressure on the hat stringer 10. Thus, the hat stringer 10 is consolidated to the fuselage skin 20 by pressure of the IML tool 30 (and the bladder 36) and curing of resin, while the caul sheet 35 controls the contour of the fuselage skin 20.
This IML fabrication method requires flawless performance of several steps. For example, the composite material must be precisely located in the IML tool cavities to avoid over or under stuffing gaps. The bladders must inflate during cure to apply compaction and cure pressure. The caul sheet must be flexible enough to account for variations in material thickness and/or ply mislocation while at the same time being sufficiently rigid to create a smooth aerodynamic surface.
An OML (outside mould line) cure process is shown in FIG. 3. These cauls panels 42, shaped like the hat stringers 10, assist in forcing the composite material into the radius during the cure cycle and in smoothing out irregularities. Thus, the fuselage skin 20 is backed by an outside mold line tool (OML) 32 and the hat stringer 10 rests on the fuselage 20, with a bladder 36 in its interior. A caul panel 42 conforming to the exterior shape of the hat stringer 10 rests on the stringer 10. A bag 40 envelopes at least the caul panel 42 and the upper surface of the fuselage skin 20. A vacuum is pulled on the bag 40 while bladder 36 is inflated and pressure is applied via IML tool 32 to consolidate the hat stringer 10 to the fuselage as the resin cures to bond the composite fuselage skin to the composite stringer 10. This caul panel-assisted co-cure method requires the fabrication and use of hundreds of uniquely designed, high maintenance, cauls and associated master tools which imposes a significant cost burden.
Accordingly, it is desirable to develop composite stringers and methods of making these that simplify processes and reduce costs. In addition, it is desirable that the composite stringers have large radii to facilitate forming the stringer out of a composite blank. Further, the stringer shapes should perform as well, or better, structurally (e.g., resist column buckling and four-point bending) than traditional trapezoidal shaped hats, when made with composite materials. In addition, there is a need for methods of making composite stringers that have smooth and gentle (larger) radii of curvature near the stringer base and that eliminate the need to use caul sheets to assist the co-cure of stringer to panel. Furthermore, other desirable features and characteristics of the composite stringers will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.